A subset of chemosensory genes differs between two populations of a specialized leaf beetle after host plant shift

Due to its fundamental role in shaping host selection behavior, we have analyzed the chemosensory repertoire of Chrysomela lapponica. This specialized leaf beetle evolved distinct populations which shifted from the ancestral host plant, willow (Salix sp., Salicaceae), to birch (Betula rotundifolia, Betulaceae). We identified 114 chemosensory candidate genes in adult C. lapponica: 41 olfactory receptors (ORs), eight gustatory receptors, 17 ionotropic receptors, four sensory neuron membrane proteins, 32 odorant binding proteins (OBPs), and 12 chemosensory proteins (CSP) by RNA-seq. Differential expression analyses in the antennae revealed significant upregulation of one minus-C OBP (Clap OBP27) and one CSP (Clap CSP12) in the willow feeders. In contrast, one OR (Clap OR17), four minus-C OBPs (Clap OBP02, 07, 13, 20), and one plus-C OBP (Clap OBP32) were significantly upregulated in birch feeders. The differential expression pattern in the legs was more complex. To narrow down putative ligands acting as cues for host discrimination, the relative abundance and diversity of volatiles of the two host plant species were analyzed. In addition to salicylaldehyde (willow-specific), both plant species differed mainly in their emission rate of terpenoids such as (E,E)-α-farnesene (high in willow) or 4,8-dimethylnona-1,3,7-triene (high in birch). Qualitatively, the volatiles were similar between willow and birch leaves constituting an "olfactory bridge" for the beetles. Subsequent structural modeling of the three most differentially expressed OBPs and docking studies using 22 host volatiles indicated that ligands bind with varying affinity. We suggest that the evolution of particularly minus-C OBPs and ORs in C. lapponica facilitated its host plant shift via chemosensation of the phytochemicals from birch as novel host plant.